In the view of the current environmental challenges there is an urgent need to develop a more sustainable chemical industry through more efficient chemical transformations and by developing new highly selective and cost-effective catalysts. One approach towards enhanced catalytic performance of supported metal catalysts is to increase the active metal surface by synthesizing small metal nanoparticles (often <10 nm in diameter). However, small nanoparticles are often prone to sintering which decreases the catalytic activity over time. The development of sinter-stable heterogeneous nano particle catalysts is therefore of great importance.
Zeolites are crystalline alumina silicate materials that exhibit a highly ordered porous structure with pores of molecular diameter. IUPAC identifies this type of porosity as microporous, as the size of the pores are not wider than 2 nm. The other groups of porosity are mesoporous (pore size between 2-50 nm) and macroporous (pore size larger than 50 nm). Zeolites consist of tetrahedral TO4 units (T=Si or Al), which gives the framework an overall composition of TO2. These materials have a clear organized framework throughout the crystals, giving rise to highly ordered pores and a large internal surface area. By replacing a silicon atom with an aluminium atom, it is possible to generate a deficit of charge, which is compensated by a cation located nearby. The cation is usually an alkali metal (such as sodium), alkali earth metal, or possibly a H+ ion. If the cation is a proton, the zeolite becomes a strong Brønsted acid. All these characteristics give rise to a lot of uses for zeolites.
Today, nearly 60 different natural occurring zeolites are known, while 201 can be prepared synthetically [1]. These zeolites have different structures, due to different Si—O—Al linkages, and a different number of Si or Al atoms linked in each “cage”. This also creates different pore system of one-, two, or three-dimensions in the zeolite.
As the pores are very regular, and around the same size in diameter as molecules, it is possible for zeolites to function as molecular sieves. Due to their chemical structure and molecular sieve properties, zeolite catalysts exhibit high selectivity for a variety of chemical reactions. Since most of the surface area and the active sites are within the zeolite, the shape of the pores and channels give rise to shape selective catalysis. Commonly there is distinguished between three types of molecular sieving effects:                1) Reactant shape selectivity: Only molecules small enough can enter the zeolite pores and undergo chemical transformation or be adsorbed.        2) Product shape selectivity: The size of the pores is too small, that not all possible products can diffuse out of the zeolite after reaction. This leads to an increased selectivity towards smaller molecules or isomers.        3) Restricted transition-state shape selectivity: Here the formation of too large transition state intermediates are prevented due to zeolite pore size. FIG. 1 illustrates the three different kinds of shape selectivity.Zeolite Synthesis        
In general, zeolite synthesis is a crystallization process, where silica and alumina species dissolve and react to give a less soluble crystalline alumina/silicate product. The crystallization process is typically performed in a hydrothermal process where the zeolite precursors is put in an autoclave and heated to relatively high temperatures and autogenous pressures. The high pressure is due to the evaporation of water inside the autoclave, and is very important for the synthesis. In a typical synthesis the zeolite precursors is dissolved or suspended in an aqueous solution of a structure directing agent (SDA) and an alkali hydroxide to catalyze the breaking and formation of chemical bonds [4].
The structure directing agents are almost always organic amine cations. Some of the most commonly used organic structure directing agents are tetramethyl-ammonium (TMA), tetraethylammonium (TEA), and tetrapropylammonium (TPA), though compounds as diverse as Choline, 1,6-diaminohexane, and hexanediol have been used. During the zeolite crystallization process, the zeolites form around molecules of the structure directing agent. The shape and properties of the structure directing agent causes the zeolites forming around it to take a certain shape. Stoichiometric analysis of samples of ZSM-5 has indicated that one TPA+ molecule occupies each intersection between pores in the zeolite [2].
For sources of silicon, mostly sodium silicate, fumed silica or tetraethoxy ortho-silicate is used, while sodium aluminate, aluminum nitrate or -chloride are typical sources of aluminum [3]. The mixture of zeolite precursors or zeolite gel is then transferred to an autoclave and heated to a predetermined temperature, often between 120-200° C. Within days, possible weeks, the precursors begin to crystallize and form the zeolite. After the synthesis, the autoclave is cooled to room temperature, and the zeolite material is washed with water and isolated by filtration or centrifugation. The zeolite is then calcined at around 500-600° C. to remove residual SDA and framework water. At last the zeolite can be ion exchanged. This can either be done to introduce hydrons, alkali metal, alkali earth metal, lanthanoid or transition metal cations.
One method to produce a porous system inside a zeolite is templating. Several types of templates have been utilised for the introduction of pores in zeolites. One of them; hard templating applies a solid material to generate a porous system in addition to the inherent micropores. This method has proved to be very effective and a highly versatile approach. Templates include organic aerogels, polymers, and carbon in different forms. Here, only carbon will be discussed. One of the well-known methods is the crystallization of zeolite gel in porous carbon particles. If the amount of synthesis gel relative to the carbon template is sufficient, the zeolite crystals continue to grow after nucleation in the cavities of the carbon. This will allow the zeolite crystal to encapsulate the carbon. Combustion of the carbon particles embedded in the zeolite crystal, will lead to the formation of mesopores [37]. Several types of carbon nanoparticles have been used [38], including carbon nanotubes [39] and nanofibers.
In 1983 Taramasso et al. incorporated titanium ions into silicalite-1 (denominated as TS-1) [56]. The incorporation of titanium, is a isomorphous substitution in the MFI lattice of the silicalite-1. The presence of a titanium atom gave rise to different catalytic properties, than the selective acid catalytic properties displayed by conventional alumina silicate zeolites. The TS-1 has been found useful in selective oxidation reactions, such as the hydroxylation of phenols, epoxidation of alkenes, and ammoxidation of ketones [57-61].
Encapsulation of metal nanoparticles in a zeolite structure can improve the physical properties of the zeolite, but in addition to that; the encapsulated metal nanoparticles can have catalytically properties themselves. As a further potential advantage, the encapsulation can protect the individual nanoparticles from contact with other nanoparticles, thereby preventing sintering of the nanoparticles when these are subjected to elevated temperatures.
In spite of the great technological, environmental and economic interests, general methods for the stabilization of metal nanoparticles against sintering are far from being fully developed, although for some specific systems it has been achieved by optimizing the interaction of nanoparticles with a support material or by encapsulation of the metal particles [52, 93, 94]. However, these known catalytic systems are in general very expensive and difficult to synthesize and they cannot be produced in industrial scale. Nanoparticles encapsulated in zeolite-like structure have only been reported in a handful of papers [46, 52, 95-99].
The encapsulation of nanoparticles is an area of increasing interest. This is a possible solution to the widely known problem of deactivation due to sintering. Several methods have been developed to produce sinter-stable nanoparticle catalyst, including encapsulating in mesoporous silica matrix or by using a protective shell [45-48]. None of these materials are however shape-selective. By encapsulating metal nanoparticles in a zeolite matrix, on the other hand, shape selective catalysis is possible. In addition, the thermal stability of zeolites and high surface area, makes zeolites particularly useful for this application. Post treatment deposition of nanoparticles inside zeolites has been reported in literature [49-51]. A limitation of these methods is however that they require zeolites containing cages.
By post-synthesis treatments the nanoparticles are in the cages and/or in the pores of the zeolite and it can be difficult to control the size and location of the nanoparticles. Both Laursen et al. and Tøjholt et al. have successfully synthesised a MFI zeolite containing gold nanoparticles (size 1-3 nm), which showed to be highly stably versus sintering [52, 53]. In addition, the gold nanoparticles were only accessible through the micropores of the zeolite. The synthesis is however difficult, and requires a lot of exotic materials and time.
So, despite the growing demand, a fast, efficient and economically process for manufacturing zeolite or zeotype encapsulated metal nanoparticles which are sinter-resistant that can be scaled up for industrial application has not yet been reported.